This invention relates to vehicle steering systems and more particularly to a centering device for controlling the steerable wheels of a vehicle such as a motor home, bus, truck, automobile or the like so that a center steering position is selected and maintained in spite of spurious steering inputs, such as those caused by variable crosswinds, crown curvature or slant of the highway, or other factors tending to adversely affect vehicle steering by the driver.
The steering systems of highway vehicles and the like are designed primarily for driver control. In these systems, the steering force required on the steering wheel and the ratio between steering wheel movement and movement of the steerable ground wheels depend upon the characteristics of the particular vehicle and the conditions under which it will usually be operated. A wide variety of extraneous forces can act on a vehicle steering system and spurious steering inputs caused by these forces must be dealt with satisfactorily in order to provide stable and controllable steering of a vehicle. As vehicle speed increases, the effects of any spurious steering inputs are magnified, making it necessary for the driver to exercise more precise and careful driving control.
Vehicles with steering systems having positive caster generally track relatively straight ahead and generally resist normal steering inputs away from center, including those of the driver. Intentional turning maneuvers by the driver therefore require sufficient turning force to overcome this positive resistance to movement away from center. When the driver relaxes the turning force applied to the steering wheel, a positive caster system has a definite tendency to return to its straight ahead position, although it may overshoot the neutral or center position if the steering wheel is entirely released.
While positive caster is desirable in some respects, it is not without compromises over the full steering spectrum. For example, the adverse effects of strong gusty cross winds are usually more pronounced with positive caster. As its name would imply, the vehicle tends to caster towards the side of the roadway to which it is being pushed by the wind. Similarly, a high crown at the center of the roadway or a slanted roadway can cause vehicles to turn toward the edge of the roadway, that is, in the downhill direction. In addition, generous positive caster provides significant resistance to small radius turns, which can make city driving quite fatiguing. These three adverse effects are some of the negative aspects of achieving steering stability through generous amounts of positive caster.
Another drawback of prior art steering systems is that spurious inputs transmitted from the roadway through the steerable wheels affect substantially the entire steering assembly before encountering any stabilizing resistance from the steering wheel. The negative action of the steerable wheels is caused by spurious steering inputs from crosswinds, slanted or crown roads, bad road surfaces, and other adverse dynamic steering forces. Inherent geometric steering characteristics may also be responsible for spurious steering inputs.
The transmission of these various inputs between the steerable wheels and the steering wheel causes the interconnecting components of the steering system to repeatedly oscillate between states of tension and compression. Such oscillations cause wear and slack in ball joints and other connections and have long been considered a primary source of stress fatigue which can lead to premature failure of various steering system components. Mechanical slack due to worn parts can also be a cause of steering system oscillations and vehicle wandering that require constant corrections and therefore produce driver fatigue.
The ideal driving situation is therefore one where the steering system inherently causes the vehicle to travel in an unswerving straight line unless the driver intentionally turns the vehicle in another direction. Thus, the ideal steering system would require relatively little attention from the driver as the vehicle progresses along a straight line path down the roadway. From a steering standpoint, the vehicle should not respond to anything but the driver""s steering commands and these must be of sufficient magnitude to overcome a significant resistance to turning away from center. In the absence of a steering input by the driver, the vehicle should literally do nothing but progress straight ahead.
The invention provides improved on-center control of the steerable wheels, and significantly reduces driver fatigue because it results in a major reduction in driver steering inputs. This is accomplished by allowing the driver to easily make small adjustments in the centered position of the steering system to fine tune steering of the vehicle during its operation. Such fine tuning is made while driving, and makes driving more pleasurable and less fatiguing.
The invention thus comprises a precision steer wheel control system having a center position which is remotely adjustable to permit the driver of a vehicle to change and reset the center position of the vehicle""s steering system to compensate for new or changed steering forces which would otherwise cause the vehicle to deviate from its straight ahead course.
In addition, the control system of the invention provides remotely adjustable levels of resistance force for opposing off-center steering movements and of centering force for returning the steering system to its center position after a turning movement. The adjustment means also provides for different levels of steering force to initiate or breakaway into a steering movement away from center. This level of force is sometimes referred to in this specification as the xe2x80x9cbreak away resistancexe2x80x9d. Different levels of break away resistance and of resistance force may be appropriate to compensate for different steering system characteristics on the same or different types of vehicles and/or for changes in the forces acting upon the vehicle. The level of break away resistance and of resistance force opposing movement away from center may be remotely adjusted either by a control mechanism operable by the driver or by a microprocessor responsive to the speed of the vehicle.
The return force for returning the steering system to center may also be remotely adjusted either by a control mechanism operable by the driver or by a microprocessor responsive to the speed of the vehicle. Both the resistance force and the return force provided by the control system are increased or decreased to provide a level of force sufficient to overcome any spurious steering inputs and to suit driver road feel, particularly a feel of the steering wheel that lets the driver know when the steered wheels are beginning to move away from center and are closely approaching return to center.
The control system may be left on continuously because it will automatically turn off with the ignition and come back on when the engine is started. With any malfunction of the vehicle""s power steering, an automatic disabling feature of the invention shuts the control system completely off.
The invention also provides a distinctive feel when approaching or leaving the center position. Thus, the sense of touch is added to the visual sense to aid control of the vehicle and reduce driver fatigue. The turning resistance selected should satisfy the road feel desired by the driver and be sufficient to overcome anticipated spurious inputs.
In the absence of the invention, spurious inputs to and/or mechanical slack in the steering assembly require almost constant manipulation of the steering wheel by the driver and make it almost impossible for the driver to hold the vehicle on a true straight ahead course. Use of the invention therefore permits a substantial reduction or elimination of the caster angle for vehicles previously requiring positive caster, thereby significantly reducing the crosswind effect and providing the driver with a positive touch control not heretofore attainable with positive caster. Positive stability is thereby achieved for previously unstable steering systems.
In addition, less manipulation of the steering system provides a substantial reduction in tire wear, particularly for large vehicles. It is estimated that use of the invention on large trucks in the United States alone may reduce tire replacement costs by as much as 3 billion dollars over a five year period.
Although the present invention is particularly useful as a centering mechanism for the steering systems of motor vehicles, it can be employed to position any steerable member moveable to either side of a preselected position. For example, the control system can keep an outboard motor centered so that a boat follows a straight course over the water in the presence of spurious steering forces produced by wind and wave action. The control system can also be used to center such steerable members as the rudders of ships or airplanes and the tongues of tandem trailers or railway cars.
The control system is useable with both power and non-powered steering systems, with the level of compensating forces provided usually being less for vehicles without power steering.
The control system may be used with steering systems having a reduction gear between the steering wheel and the steerable wheels. In this application, the control system is preferably connected to the steering system at a location between the steerable wheels and the reduction gear so as to be unaffected by any slack in the reduction gear or in components and connections between the reduction gear and the steering wheel. It is therefore on the slow side of the reduction gear ratio. The invention thus provides a zero backlash hydraulic centering assembly.
The centering assembly of the control system is preferably connected between the steering system and the front axle or a frame member of the vehicle in a position that allows the steerable member to move through its full range of steering movements while providing sufficient leverage for the assembly to resist movement of the steerable member away from the center position producing straight ahead travel of the vehicle. The steering system connection may be made to any steering system component providing appropriate range and leverage, such as a tie rod which joins the two front steerable wheels of a highway vehicle, or the pitman arm connected to the reduction gear. The frame connection may be made to any component serving as a fixed mounting relative to the steering system. This fixed component may be a frame member, or an axle or some other part carried by the vehicle frame instead of an actual frame member.
The preferred embodiments of the invention therefore have the following features: resistance force for opposing steering movement away from center and return force for return to center each of sufficient magnitude to overcome spurious steering inputs, remotely variable levels of resistance force and return force through a predetermined range of turning movement, remotely trimmable center position, no substantial overshoot, free return to center position, fail-safe mode for disabling the control system in the absence of power steering, precision and slack-free centering, slack free trimming, dampening of erratic steering movements, compact size, utility for old and new vehicles with or without power steering, and distinctive driver road feel.
It follows that a principal object of the present invention is to provide a power centering control system having the foregoing features and attachable to a vehicle steering system to reduce automatically the driver inputs required to maintain a preselected straight ahead course for the vehicle. To accomplish this, the present invention offsets spurious steering inputs with an opposing resistance force sufficient to keep the steerable wheels or other steering member in a selected center position for maintaining straight ahead vehicle travel. Accurately holding the steerable wheels on center prevents road wander, slanted road steering wheel pull, steering over-control, crosswind steering effect, and steering induced dynamic sway. Improvement in rutted road steering may also be realized. In addition, the positive centering feature gives the steering system designer greater latitude in reducing or eliminating the caster angle of the kingpin, which will reduce or eliminate the crosswind steering effect.
The manner in which the present invention accomplishes the foregoing objectives and advantages will now be described. The resistance force is provided by a zero backlash hydraulic centering assembly that is preferably attached at one end to a fixed frame member and at the other end either to the steering gear pitman arm or directly to the steering system tie rod. The assembly comprises a centering cylinder containing two pistons having rest positions against a center stop that holds an enlarged head of a piston rod captive in an on-center position. When the steered wheels are turned away from center, one of the pistons is displaced by the piston rod head. When the driver releases the steering wheel, the displaced piston returns the piston rod head and the steering system to their on-center positions. A hydraulic pressure source maintains substantial pressure in cylinder chambers on one side of each piston and this pressure causes the captured piston rod head to keep the steered wheels on center, tracking with accuracy that is not achieved with any other method.
The hydraulic pressure source is preferably an air over hydraulic pressure accumulator that includes a reservoir for the hydraulic fluid. Air pressure from a pressurized air source, such as an onboard compressor, a vehicle airbrake system or some other conventional air pressure source, is regulated by a control on the driver control panel. The accumulator has a flexible diaphragm that defines separate air and liquid chambers. By selectively varying the pressure within the air chamber, hydraulic fluid pressure and resulting forces applied to the dual pistons are varied, thereby varying the resistance to off-center movement of the steering system, as well as the return force for recentering the steering system. A pressure relief valve may communicate with the air chamber side of the accumulator to provide an upper limit to the resistance and return forces that may be generated by contact between the respective pistons and the piston rod head therebetween.
The turning resistance of the present invention is therefore provided by delivering fluid under pressure to the centering cylinder from a fluid pressure accumulator which may take a variety of forms and may be hydraulic, pneumatic or a combination of both. A pressure control device permits the level of resistance to movement away from center and the level of return force to be controllably varied, either by a hand mechanism operable by the driver or by a speed control mechanism responsive to the speed of the vehicle. In a preferred embodiment of the invention, the centering cylinder is preferably pressurized by liquid from the accumulator, and liquid in the accumulator is preferably pressurized by an air system of the vehicle.
The pressure accumulator may be protected from moisture by suitable air filters and/or dryers. Dual electric pneumatic valves, one on top and one on the bottom of the accumulator, automatically dump the air pressure when the system is turned off. The air pressure in the accumulator is automatically recharged each time the system is turned on, thereby eliminating a recharge maintenance function. On systems supplied by an air compressor, the discharge from the upper accumulator dump valve may be routed to the compressor inlet filter for providing a backflush function to keep this compressor filter clean.
It is also within the scope of the present invention to pressurize the centering cylinder using other liquid or air pressurization systems. For example, a pressure accumulator system of the type described in my prior U.S. Pat. No. 4,410,193 may be employed for storing and providing hydraulic fluid under pressure to the centering cylinder. Regardless of the type of fluid pressurization system employed, the system should generate sufficient pressure to return the respective centering pistons to their center rest positions against the central centering stop of the centering cylinder upon cessation of intentional steering inputs. The pressurization system should also constantly bias the centering pistons into engagement with both the centering stop and the piston rod head at all times when the control system is activated and in its center position so that there is no slack in the control system linkages at any time during its operation.
Spurious steering inputs tending to move the tie rod in either direction are therefore resisted by a corresponding resistance force generated by interaction between a corresponding piston and the piston rod head. Only when intentional steering wheel forces exceed a selected break away level will the tie rod of the steering system generate sufficient linear force on the piston rod to cause its head to move off center along with the piston corresponding to the direction of the desired turning movement.
A remotely operable trimming means is provided for controllably varying the selected center position of the steerable member to be maintained by the control system. In particular, a trim rod connected to a trim piston is arranged for movement in either direction within a hydraulic trim cylinder. The distal end of the trim rod is pivotally connected to either the steering system or the vehicle frame, depending on which of these is opposite from the distal end connection of the centering rod. Hydraulic fluid from the fluid system accumulator is supplied to opposite sides of the trim piston and its flow is controlled by a trim valve means operated by a single trim solenoid to allow movement of the trim piston and its rod.
A particularly important feature of the present invention is the location of the trim solenoid, which is positioned within the innermost centering chamber where it is mounted on an intermediate head between this chamber and the innermost trim chamber. A special recess is provided in the innermost centering piston to receive the projecting structure of the solenoid and thereby prevent any interference between this structure and compressive movement of the piston. Compared to centering assemblies with externally mounted solenoids, such as described in my prior U.S. Pat. No. 5,536,028, this arrangement greatly increases the compactness and symmetrical shape of the assembly, allowing it to be more easily handled and installed in a smaller space.
The trim cylinder is separate from, but integrally attached to, one end of the centering cylinder. The trim piston preferably can move about one-half inch to about one inch to either side of its center position in the trim cylinder, i.e., the total stroke of the trim piston is preferably in the range of about one inch to about two inches. The hydraulic fluid in this short trim cylinder is trapped on opposite sides of the trim piston by the trim valve when it is closed, creating a hydraulic lock that holds the centering stop of the centering cylinder in a selected on-center position.
When an electric trim button is activated, the solenoid operated trim valve opens, allowing fluid to pass from one side of the trim piston to the other. This causes the centering cylinder (and its centering stop) to move precisely by the amount necessary to coincide with the straight ahead direction that the vehicle is being steered. Releasing the trim button allows the trim valve to close, recreating a hydraulic lock in the trim cylinder to hold the centering assembly in the new on-center position to which it has been trimmed. The average trim corrections may be on the order of a few one-thousandths of an inch. A liquid trimming fluid is preferred because it is substantially incompressible as compared to a gaseous trimming fluid and therefore provides the capability of locking the trim piston in its trimmed position without appreciable slack.
A driver control panel makes it possible for steering corrections to be made while driving. The panel may be conveniently located near the driver and provides three basic functions, namely, a switch to turn the system on and off, a centering effectiveness control, and a trim control such as a momentary trim switch. Activating and adjusting the centering force and trimming the centering system is therefore an easy and natural driving function. Should the driver sense a degree of steering wheel pull that becomes a bother, it is then quickly eliminated by momentarily pressing the trim switch.
The apparatus includes control means for remotely and selectively varying both the amount of resistance to movement away from center and the selected center position of the steerable member(s) relative to the vehicle frame. Both of these remote adjustments are preferably made by the driver while the vehicle is in operation. Therefore, electrical control and fluid control systems are employed for remotely operating the centering and trimming units of the centering assembly from the driver""s station of the vehicle. The electrical controls may comprise one or more switches preferably having a toggle design that is spring-biased to a circuit-open position. Such switches are closed only momentarily when the toggle is held in a depressed position against the spring bias. Thus, the internal trim solenoid is actuated only while the toggle is depressed. Release of the toggle opens the circuit and stops the trim adjustment at the point selected.
My earlier U.S. Pat. No. 4,410,193, No. 4,418,931, No. 4,534,577 and No. 5,536,028, the entire contents of which are expressly incorporated herein by reference, solved many of the shortcomings existing in the prior art. The present invention provides improvements over these earlier patents, especially in the areas of compactness, ease of handling, simplicity, precision, and reliability.